This invention relates to a centralizer for the stem of an intramedullary prosthesis such as a femoral implant. More particularly it relates to a centralizer with deformable tangentially extending spring elements in the form of wings or fins.
Centralizers for example, as shown in British Patent Application 2 104 391 are used to centralize the distal stem in the femoral canal. A disadvantage with devices of the type shown in U.K. 2 104 391 is that the resilient elements which are provided to centralize the tip end of the stem of the prosthesis can only be pressed inwardly to a certain extent and difficulties can arise if the intramedullary canal is particularly narrow. Another centralizing device using vertically extending elements is shown in U.S. Pat. No. 3,793,650 which issued to the inventors of U.K. patent application 2 104 391. A polyethylene sleeve is shown in the Exeter Universal hip system similar to that of 3,793,650 in that the spring elements extend vertically. It will be appreciated that these spacers are not only intended to centralize the distal tip of the prosthesis but also to prevent it from actually engaging a wall of the canal.
A further disadvantage with the prior art devices is that, when used in cemented applications, they tend to leave voids and air bubbles in the cement behind them as they are pushed downwardly and these voids and air bubbles also form behind the fingers, that is, between the fingers and the body of the device. This is to be avoided as it can become a point of weakness and crack initiation within the cement mantel.
Polymethylmethacrylate sleeves are known and have been taught by Leo A. Whiteside et al in an article published in June 1988 entitled "The Effects of the Collar on Total Hip Femoral Component Subsidence" and sold as the "Whiteside Total Hip System". However, this device is merely a cylindrical sleeve which can only centralize the distal stem in a precisely reamed canal.
Also, it is advantageous to provide a centralizer on which in certain circumstances the forces causing subsidence, such as a heavy load on the prosthesis, are high enough to cause the stem to move further into the cement mantle and thus further into the centralizer, which offers the advantage of being able to absorb this movement by providing means to accept the movement and cover the end of the stem to thus prevent it piercing further into the cement and causing cracking.
The centralizer of the present invention overcomes the problems set forth above. According to the present invention, a centralizer for the stem of a prosthesis for introduction into an intramedullary canal comprises an annular body portion for location on the distal tip of the stem of a prosthetic component. The body portion is provided with three or more resiliently deformable spring elements in the form of fins or wings projecting outwardly from the centralizer body portion which are adapted to fold circumferentially and inwardly towards the body portion.
Thus, with this construction the fins are deflected inwardly towards the body portion as the stem enters the narrower part of a canal. In addition, due to the fact that the fins can be made of relatively thin sections, they cut their way through the bone cement as the stem is introduced into the canal without having to bulldoze their way through the cement leaving voids in their wake. Thus, voids and air bubbles are prevented or reduced.
A further advantage of the centralizer of the present invention is that its thin fins or wings can collapse inwardly down to a diameter of less than 10 mm and therefore the surgeon will less frequently be faced with the need to trim parts or all of the fins or wings off the body portion. This extra step has sometimes been necessary with constructions as set forth in the U.K. Patent Application 2 104 391 referred to above.
Preferably the fins or wings extend outwardly from the body portion in tangential directions so that they are lined up ready for deformation in the correct direction. The body portion can be provided with control elements to allow the distal tip of the implant to move further into the centralizer after fitting and after the prosthesis and centralizer have been cemented into place. With this arrangement, the body portion is preferably shaped to enclose the tip end of the prosthesis stem.